Distillation of a fluid mixture, e.g., air, into two or more portions enriched in a respective mixture component, has generally been carried out employing one or more distillation columns which employ trays as the column internals or mass transfer elements. Recently there has developed an increasing use of packing as mass transfer elements in distillation columns because packing has a much lower pressure drop than does trays. Packing may be either structured or random packing. Structured packing is particularly preferred because it has more predictable performance than does random packing.
While packing has advantages over conventional trays in the operation of a distillation column, there has been experienced with packing a significant reduction in separation efficiency over that which would be expected theoretically. Applicants believe that these separation performance deficiencies are the result of liquid bypassing within a given packed section by flowing along the column wall. The path of least resistance for liquid in a packed column is down the wall of the column. Wall liquid has a much shorter residence time in the column than the bulk liquid which flows through a torturous path within the packing. Mass transfer between the wall liquid and bulk vapor will be less than the corresponding mass transfer between the bulk liquid and the bulk vapor. Hence, as one proceeds down the column wall of a distillation column, wall liquid will contain more of the low boiling or more volatile component relative to the bulk liquid. For example, in a typical cryogenic oxygen plant for the column section located above the main condenser separating a binary mixture of oxygen and argon using a column with structured packing, above the packing is a distributor containing liquid comprising from about 10 to 20 percent argon with the balance being primarily oxygen. Liquid flows from the distributor in a uniform fashion to the first element of packing. An element in this case being a 10 inch high layer of structured packing having corrugations that run at a 45.degree. angle from the column axis. Some of this high argon content liquid is diverted by way of the corrugations to the column wall. This wall liquid flows in relatively fast moving rivulets with minimal surface area. These conditions lead to poor mass transfer efficiencies. The bulk liquid on the other hand spreads into thin slow moving films of substantial surface area which are ideal conditions for efficient mass transfer. Consequently, the wall liquid adjacent to the bottom of the first element will contain more argon, the low boiling component, than the bulk liquid leaving the packing. At the next element down, a portion of the wall liquid flows back into the packing. Some mass transfer occurs between the remaining wall liquid and the bulk vapor and some liquid of slightly lower argon concentration is added to the wall flow. The amount of wall liquid relative to bulk liquid increases as it descends. Again the wall liquid adjacent to the bottom of the second element will contain more argon than the bulk liquid leaving the packing. This process continues past subsequent elements to the bottom of the packed section. At this elevation, the wall liquid having substantial argon concentration compared to the bulk liquid, mixes with the bulk liquid leaving the packing. The argon concentration of the mixture is elevated compared to the bulk liquid so the apparent separation for this section is poor.
There are known in the art means for correcting liquid maldistribution within a packed column. For example, there has been proposed the use of liquid redistributors or trays between short packed column sections to collect and mix wall liquid with bulk liquid. However, the capital and process cost penalties are substantial. Further mixing of the wall liquid with the bulk liquid represents a thermodynamic irreversibility. Furthermore, costs are incurred due to the need for additional bed supports, collector trays, which collect wall liquid and mix it with bulk liquids, and a liquid distributor which is fed by the collector tray, the distributor providing uniform flow of the liquid to the subsequent packed bed. Also, additional column height is required, the cost of which includes additional welds, column material, insulation, ladders, platforms, etc. The foundation is ultimately effected. Another concern is the transfer of liquid within the plant when columns get too tall. Along with capital penalties there are process penalties involved with redistribution. In particular, redistributors add vapor phase pressure drop which must be overcome by additional feed pressure. Furthermore, redistributors do not add to the mass transfer potential of the column. From a fundamental perspective, a redistributor serves to mix liquid containing a relatively high concentration of the low boiling component (wall liquid) with the bulk liquid which contains the relatively low concentration of the low boiling component. The mixed liquid then undergoes further distillation in a subsequent packed section to reduce the concentration of the low boiling component in the liquid. The liquid mixing that occurs in a redistributor represents a thermodynamic irreversibility with a quantifiable process penalty. Combining this process penalty with the practical drawbacks and process penalties of liquid redistribution cited earlier provide substantial incentive for developing alternative means for dealing with wall flow.
Accordingly, it is an object of this invention to provide an improved packed column distillation system which can overcome the problems of liquid maldistribution cause by wall flow, without the need to resort to liquid redistributors or trays between packed column sections to collect and mix wall liquid with bulk liquid.